fixed: ERT depends on libdl

this issue was triggered with static libs for ERT
ERT: link to the threads library of the OS

this is required to make the opm-core build succeed if ERT was build
with -DBUILD_SHARED_LIBS=OFF . (without it, I get errors like

    /home/and/src/ert/devel/libert_util/src/thread_pool_posix.c:328: error: undefined reference to 'pthread_create'
Add refDepthDefaulted field to Well class.
Adapt tests to use new Well constructor, add test for defaulted ref depth.
Change Well constructors to avoid explicit bool arg.

Suggested by Joakim Hove.
Update checkWELSPECSConsistency() method.

Now accounts for new refDepthDefaulted() method and semantics.
Merge pull request #274 from atgeirr/default-wellrefdepth

Correctly handle defaulted well reference depths
Merge pull request #273 from andlaus/allow_static_ERT

Allow static ert
add the FGIT keyword to the FIELD_PROBE

This is used by Norne, but for some reason it was not included in the
deck_names list...
make the regex test more thorough

it seems like the std::regex delivered up to gcc 4.8 has problems with
constructs such as ".+". (at least if they appear at the end of a
regex...)
Merge pull request #275 from andlaus/add_FGIT

add the FGIT keyword to the FIELD_PROBE
Merge pull request #276 from andlaus/better_regex_test

make the regex test more thorough
Added new EclipseGrid constructor

The new EclipseGrid constructor EclipseGrid(nx,ny,nz , GridSection)
allows the construction of a GRID without the RUNSPEC section.
Merge pull request #280 from joakim-hove/EclipseGridConstructor

Added new EclipseGrid constructor
Add dimension (length) to THRESHOLD_THICKNESS item.
Also add dimension to MAX_EMPTY_GAP property.
Merge pull request #281 from atgeirr/pinch-threshold-dimension

Add dimension (length) to THRESHOLD_THICKNESS item.
Add isPinchActive() and getPinchThresholdThickness() members.

Now reading the optional PINCH keyword from the GRID section.
Add test for pinch features of EclipseGrid.
Make getPinchThresholdThickness() throw if isPinchActive() is false.